1. Field of the Invention
The present invention relates to unbalanced (i.e. single-ended signal) to balanced signal (i.e. differential signal) conversion. More particularly, the invention relates to a matching network converting from an unbalanced signal input to a component having a balanced input.
2. Background of the Related Art
A “balun” is a circuit element or a collection of circuit elements that transforms an unbalanced signal into a balanced signal. The balanced signal has two components, wherein a first and second component are related to the unbalanced signal but are mutually substantially opposite in phase (i.e. a 180° phase shift).
An exemplary use of a balun may be found in a radio receiver, which usually includes a low noise amplifier (LNA) for receiving signals from an antenna. An LNA is frequently implemented in a balanced-input (i.e. differential) configuration, especially when implemented on an integrated circuit. An antenna is commonly a source of unbalanced signals, thereby requiring conversion to feed an LNA.
Transformers are useful for this conversion, wherein an unbalanced input can be connected to one side of a first winding and the other side of the first winding can be connected to ground. For example, FIG. 1A illustrates a balun circuit 100 having an unbalanced input 110 connected to a first winding of a transformer 111 (which is further connected to ground). A second winding of transformer 111 then presents two signals to a balanced-input LNA 118. Therefore, transformer 111 serves as a balun for balanced-input LNA 118. In this embodiment, capacitors 115 and 116 are DC-blocking coupling capacitors of relatively large value (e.g. 39 pF) that connect the second winding of transformer 111 to the positive and negative terminals of LNA 118. Unfortunately, transformers are relatively expensive components, especially transformers suitable for use at higher radio frequencies (e.g. 1.9 GHz). Therefore, as microelectronics decrease in cost, the cost of transformer 111 becomes commercially non-viable for a manufacturer.
Note that it is possible to simply feed the antenna input to one side of a balanced-input LNA. For example, FIG. 1B illustrates a balun circuit 120 having unbalanced input 110 connected through a coupling capacitor 123 and a matching network 124 (including a matching capacitor 125 and inductor 126) to the negative input terminal of LNA 118. The positive input terminal of LNA 118 is terminated through a coupling capacitor 122 and a resistor 121 to ground.
Although balun circuit 120 is less expensive than balun circuit 100 (FIG. 1A), resistor 121 can generate undesirable thermal noise. Moreover, the difference in circuit structure on each of the input terminals in balun circuit 120 raises the LNA's susceptibility to common mode noise. Therefore, balun circuit 120 can fail to provide robust common-mode signal rejection, which is a noise mitigating property of balanced circuits.
Thus, a typical implementation of balun circuit 120 generates a higher noise figure (NF) than balun circuit 100 (FIG. 1A). For example, in one typical implementation of balun circuit 120 operating at 1.9 GHz (e.g. capacitors 122, 123, 125 having capacitances of 39 pF, 39 pF, and 1.5 pF, respectively, resistor 121 having a resistance of 50 Ohms, and inductor 126 having an inductance of 4.7 nH), a noise figure (NF) of approximately 4 dB was measured from unbalanced input 110 to the output of LNA 118 compared to the above-described implementation of balun circuit 100 that generated only 3 dB.
Therefore, a need arises for a method and an apparatus that can provide a low-cost, low-noise unbalanced to balanced conversion for an LNA.